This invention relates to a heating apparatus equipped with a sensor element such as a humidity sensor, for controlling the heating.
Conventional heating apparatus, for example, high-frequency heating apparatus such as a microwave oven exhibit such operational difficulties that the duration of heating of a substance to be heated is variable depending on the amount of the substance. A heating failure such as overheating or non-uniform heating tends to occur unless the high-frequency output level is suitably switched over depending on the kind of substance to be heated. In view of the above operational difficulties, automation of the heating apparatus has been attempted in which the heating duration and the high-frequency output level are not preset, and a sensor element such as a temperature sensor, an infrared sensor or a humidity sensor is employed for automatically sensing the time to terminate the heating process. However, in view of the fact that these sensors have individual advantages and disadvantages, the user had to manipulate the apparatus in such a way as to suitably compensate for the disadvantage of the sensor employed in the apparatus.
In the case of, for example, a temperature probe containing a temperature sensor in one end of a rod-like metal tube, it has the advantage of successfully sensing the temperature of an inner central portion of a substance being heated although it is most difficult to sufficiently heat that portion. On the other hand, the temperature probe is not effective in sensing non-uniform heating of the substance, and a heating failure tends to occur in which the surface portion of the substance has been carbonized when the temperature of the inner central portion of the substance attains the desired level. Further, the selection of the area of the substance into which the temperature probe is to be inserted is still left as one of the key points in achieving successful cooking.
In contradistinction to the temperature probe, an infrared sensor can merely sense the surface temperature of a substance being heated, and it is indispensable to estimate the heating duration on the basis of the amount of the substance to be heated. Thus, automation is difficult more or less although the fact that this sensor is a non-contact type of sensor is attractive.
A humidity sensor senses primarily water vapor generated from a substance being heated. The result of humidity sensing by the humidity sensor is free from appreciable errors since a large amount of water vapor is not generated until both the temperature of the surface portion of the substance and the temperature of the inner central portion rise up to a certain level. However, unless the relative humidity of the air in the heating chamber of the apparatus varies greatly, the humidity sensor does not accurately sense the time of vapor generation, and the heating will continue without ending. It is therefore essentially necessary to hermetically cover the substance with a wrap of plastic film or like material. When the vapor pressure within the wrap covering the substance attains a level higher than a certain level, the vapor blows out into the heating chamber by thrusting through the wrap, and the relative humidity of air in the heating chamber varies greatly. In that state, the function of the heating sequence control by the humidity sensor is attained with higher reliability.
FIG. 1 is a graph illustrating the effect of such a wrap, by way of example. In FIG. 1, the broken curve A represents the relative humidity of air in the heating chamber when the wrap is not provided, while the solid curve B represents the relative humidity when the wrap is provided. Water vapor starts to generate from the substance at time T.sub.n. Till that time T.sub.n, the relative humidity shows a decreasing tendency since there is neither increase nor decrease in the absolute quantity of humidity of air in the heating chamber, and on the other hand, the internal temperature of the heating chamber is increasing steadily. When the substance is not covered with the wrap, a slight quantity of vapor emanates locally but continuously from the surface of the substance resulting in a slow but gradual increase in the relative humidity of air in the heating chamber. Therefore, the variation .DELTA.H.sub.A of relative humidity between time T.sub.n and time (T.sub.n +.DELTA.T) is not so large. When the humidity sensor is used for the purpose of heating sequence control, therefore, it is indispensable to cover the substance such as a foodstuff with the wrap of plastic film or like until the vapor generation time T.sub.n is reached. However, the finished state of the foodstuff heated while being covered with the wrap is analogous to that of a steamed foodstuff, and it is necessary to remove the wrap at the time T.sub.n when it is desired that the food, such as a roast, have a crisp finish.
The individual sensors have thus individual advantages and disadvantages, and the user of the apparatus had to master the way of skillfully handling the apparatus which is equipped with one of the sensors having such advantages and disadvantages.
It is the object of the present invention to provide a heating apparatus having a voice synthesizer system which is capable of announcing to the user instructions or advice by a voice message at the proper times on the basis of data output from a sensor such as a humidity sensor.